The ultimate goal of many wireless providers of communication services is to combine high bandwidth data and video services into a single platform with more conventional and lower bandwidth voice services. To achieve this goal, more limited technologies associated with conventional voice dominated networks are being replaced with much newer and state-of-the-art technologies that support higher information bandwidths. Data transmission protocols such as Frequency Division Multiple Access (FDMA) and Frequency Division Duplex (FDD) were once highly innovative technologies when applied to traditional voice-only networks. FDD supports two-way radio communication by using two distinct radio channels. In a fixed wireless point-to-point system that uses FDD, one frequency channel is transmitted downstream from a radio “A” to a radio “B” and another second frequency channel is used in an upstream direction to support transmission from radio “B” to radio “A”. Because of the pairing of distinct frequencies, simultaneous transmission in both the upstream and downstream directions is possible, so long as a sufficient amount of frequency separation (i.e., guard band) is maintained to limit self-interference. As shown by FIG. 1A, this guard band is illustrated as a channel spacing between adjacent channels, a transmit channel (left) and a receive channel (right).
FDD systems can be used for asymmetric downstream and upstream traffic when the downstream and upstream channel bandwidths are matched precisely to the asymmetry. Nonetheless, FDD systems are most suited for symmetric traffic applications in which the same or similar information flows in both directions, such as voice communications.
Fortunately, there are now newer technologies on the market that support the higher bandwidth demands and more dynamic nature of networks that deliver a combination of voice, video, internet and other data services more efficiently by supporting “bursty” and asymmetric traffic (e.g., Internet). For example, Time Division Duplex (TDD) uses a single frequency channel to transmit signals in both the downstream and upstream directions by transmitting the signals in different time “slots.” TDD operates by toggling transmission directions (at high speed) over a time interval, which means that TDD can support voice and other symmetrical communication services as well as asymmetric data services, and can even support a dynamic mix of both traffic types. In addition, the relative capacity of the downstream and upstream links can be altered in favor of one direction over the other direction. This is easily accomplished by providing a greater time allocation through additional time slots to downstream (or upstream) transmission intervals relative to upstream (or downstream) transmission intervals. This asymmetric capability is useful for communication processes characterized by unbalanced information flow, such as when a relatively short upstream message prompts a large information download as is typical with Internet access.
To support the use of a single frequency channel, TDD systems require a guard time interval (instead of guard frequency band) between transmit and receive data streams, as shown by FIG. 1B. This guard time interval, which is often referred to as a TX/RX transition gap (TTG), enables a base station to switch from transmit mode to receive mode and subscribers to switch from receive mode to transmit mode. During this gap, the base station and subscriber are not transmitting modulated data but are simply allowing the base station transmitter carrier to ramp down, the TX/RX antenna switch to actuate and the base station receiver section to activate.
Moreover, because in TDD, both the transmitter and receiver operate on the same frequency band (but at different time intervals), TDD systems can reuse filters, mixers, frequency sources and synthesizers, which contrasts with FDD systems since in those systems the equipment operates at different frequencies. TDD can also support interference mitigation via proper frequency planning because TDD requires only one interference-free channel compared to FDD, which requires two interference-free channels.